The subject invention pertains to ethylene polymer compositions which are useful in film applications. In particular, the subject invention pertains to ethylene polymer compositions which exhibit the processability of highly branched low density polyethylene, while exhibiting improved mechanical properties, and to films prepared therefrom.
Historically speaking, highly branched low density polyethylene has found great utility in blown film applications, attributable in part to its unique processability. Large amounts of long chain branching and a broad molecular weight distribution give this polymer the shear thinning and melt strength properties unmatched by heterogeneously branched linear low density polyethylene resins. Non-Newtonian shear thinning provides the high shear, low melt viscosity for good extruder processability and low shear, high melt viscosity for superior blow film bubble stability.
Low density polyethylene has found utility in a variety of film applications. Markets which require a combination of high processability resins, but do not require high film clarity, include industrial liners, heavy duty shipping sacks, non-clarity rack and counter bags, mulch film, and rubber separators. Markets which require a combination of high processability resins and high clarity films include clarity liners, bakery films, shrink films, and garment bags.
The performance requirements vary depending upon the application, but include elements of (1) the polymer xe2x80x9cextrudabilityxe2x80x9d (high shear rheology) and melt strength (low shear rheology); (2) mechanical properties of the fabricated article; and (3) optical properties of the fabricated article. The actual performance requirements are given in terms of (1) the film bubble stability, polymer output rate (kg/hr) and extruder performance (pressure, melt temperature and motor amperes); (2) strength of the fabricated article (such as tensiles, resistance to tear, resistance to puncture); and (3) clarity, haze and gloss of the fabricated article.
Heterogeneously branched ethylene/xcex1-olefin interpolymers, which are referred to in industry as linear low density polyethylene (LLDPE), have likewise found utility in blown film applications. In many respects, such resins are preferred to low density polyethylene, as they lead to blown films exhibiting tear and toughness properties. However, such polymers are more difficult to process and have decreased optical properties, such as haze and clarity, than films prepared with highly branched low density polyethylene.
In developing markets, demand for polyolefins which exhibit the processability of low density polyethylene is growing. However, the demand is currently outpacing the investment in new low density polyethylene plants. The industry would find advantage in olefin polymer compositions which are useful to prepare blown films which have toughness and impact properties comparable to heterogeneously branched ethylene/alpha-olefin interpolymers, which exhibit the processability and optical properties of highly branched low density polyethylene. Preferably, such polymer compositions would be produced in low pressure solution, slurry, or gas phase polymerization reactions.
U.S. Pat. No. 5,539,076 discloses a particulate polymer composition which is an in situ catalytically produced blend having a broad bimodal molecular weight distribution. Molecular weight distributions of 2.5 to 60 are broadly claimed, with molecular weight distributions of 10 to 50 being preferred, and of 15 to 30 being most preferred.
U.S. Pat. No. 5,420,220 discloses a film comprising a metallocene-catalyzed ethylene polymer having a density of from 0.900 to 0.929 g/cm3, an I21/I2 of 15 to 25, an Mw/Mn of from 2.5 to 3.0, and a melting point ranging from 95xc2x0 C. to 135xc2x0 C. A polymer having an I21/I2 of 18 and an Mw/Mn of 2.6 is exemplified.
U.S. Pat. No. 4,205,021 discloses a copolymer of ethylene and a C5-C18 xcex1-olefin, which copolymer has a density of from 0.90 to 0.94 g/cm3. The disclosed compositions are said to have long chain branching, and are described as preferably having two or more DSC melting points. U.S. Pat. No. 4,205,021 discloses the use of the disclosed polymers in blown films.
U.S. Ser. No. 08/858,664 filed May 5, 1997 (PCT Publication WO 93/13,143), discloses the in-situ preparation of a blend of two ethylene polymers prepared with a constrained geometry catalyst, wherein each of the polymers is said to have a melt index (I2) of from 0.05 to 50 g/10 minutes. The polymers may be prepared in a single reactor with two active catalyst species, or may be produced in a dual reactor configuration with either the same or different constrained geometry catalysts being provided in each reactor.
The industry would find advantage in olefin polymer compositions which will usefully replace high pressure low density polyethylene, without requiring film fabricators to engage in significant reconstruction and retrofitting of their fabrication lines. The desired olefin polymer compositions should have processability and optical properties which are at least roughly equivalent to that of highly branched low density polyethylene. Preferably, the desired olefin polymer compositions will further exhibit toughness and impact properties which are improved over the properties of low density polyethylene. Preferably, such polymer compositions will be produced in low pressure solution, slurry, or gas phase polymerization reactions.
Accordingly, the subject invention provides a film having at least one layer comprising an interpolymer of ethylene and at least one comonomer selected from the group consisting of C3-C20 xcex1-olefins, dienes, and cycloalkenes, wherein the interpolymer is characterized as having:
a. a density of from 0.910 to 0.930 g/cm3,
b. a melt index (I2) of from 0.2 to 10 g/10 minutes,
c. an I10/I2 of from 9 to 20, and
d. a molecular weight distribution, Mw/Mn of from 2.1 to 5.
In an especially preferred embodiment, such a polymer will further have from one to two crystallization peaks as determined by TREF, each occurring between 45xc2x0 C. and 98xc2x0 C., with each having a CTBI of less than 18xc2x0 C.
In one preferred embodiment, the interpolymer will have an I2 of from 1.0 to 7 g/10 minutes. In a more preferred embodiment, the interpolymer will be prepared in two polymerization reactors, each of which contains a single site constrained geometry or metallocene catalyst. In such a more preferred embodiment, the interpolymer, upon fractionation by gel permeation chromatography, will most preferably be characterized as comprising:
a. from 25 to 90 percent of a first polymer fraction having a melt index (I2) of from 0.05 to 1.0 g/10 minutes, and a single crystallization peak between 45xc2x0 C. and 98xc2x0 C. having a CTBI value of less than 18xc2x0 C. as determined by TREF; and
b. from 10 to 75 percent of a second polymer fraction having a melt index (I2) of at least 30 g/10 minutes, and a single crystallization peak between 45xc2x0 C. and 98xc2x0 C. having a CTBI value of less than 18xc2x0 C. as determined by TREF.
In another preferred embodiment, the polymer will have an I2 of from 0.05 to less than 2.5 g/10 minutes, an I10/T2 of at least 12.5, and an Mw/Mn of from 2.1 to 3.0. In this alternate preferred embodiment, the polymer will most preferably be characterized as having a single crystallization peak between 45xc2x0 C. and 98xc2x0 C. having a CTBI of less than 18xc2x0 C. as determined by TREF.
The subject invention further provides a process for preparing a blown film comprising:
a. melting an interpolymer to a temperature of 300 to 350xc2x0 F. (149 to 177xc2x0 C.),
b. extruding the interpolymer at the rate of 15 to 50 lb/hr (6.8 to 23 kg/hr) through a die having a 40 to 80 mil (1 to 2 mm) die gap,
c. blowing the film to into a bubble, at a blow-up-ratio of 1.3 to 2, to form a 0.5 to 4 mil (0.01 to 0.1 mm) gauge film, and
d. cooling the film by means external to the bubble,
wherein the interpolymer is an interpolymer of ethylene and at least one comonomer selected from the group consisting of C3-C20 xcex1-olefins, dienes, and cycloalkenes is characterized as having:
i. a density of from 0.910 to 0.930 g/cm3,
ii. a melt index (I2) of from 0.2 to 10 g/10 minutes,
iii. an I10/I2 of from 9 to 20, and
iv. a molecular weight distribution, Mw/Mn of from 2.1 to 5.
In an especially preferred process, the interpolymer employed will have from one to two crystallization peaks between 45xc2x0 C. and 98xc2x0 C., each having a CTBI of less than 18xc2x0 C., as determined by TREF.
The subject invention further provides a process for preparing a blown film comprising:
a. melting an interpolymer to a temperature of 300 to 400xc2x0 F. (149 to 204xc2x0 C.),
b. extruding the interpolymer at the rate of 15 to 50 lb/hr (6.8 to 23 kg/hr) through a die having a 40 to 80 mil (1 to 2 mm) die gap,
c. blowing the film to into a bubble, at a blow-up-ratio of 2 to 4, to form a 2 to 5 mil (0.05 to 0.13 mm) gauge film, and
d. cooling the film by means external to the bubble,
wherein the interpolymer is an interpolymer of ethylene and at least one comonomer selected from the group consisting of C3-C20 xcex1-olefins, dienes, and cycloalkenes is characterized as having:
i. a density of from 0.910 to 0.930 g/cm3,
ii. a melt index (I2) of from 0.05 to 2.5 g/10 minutes,
iii. an I10/I2 of from 12.5 to 20, and
iv. a molecular weight distribution, Mw/Mn of from 2.1 to 3.
In an especially preferred process, the interpolymer employed will have from one to two crystallization peaks between 45xc2x0 C. and 98xc2x0 C., each having a CTBI of less than 18xc2x0 C., as determined by TREF.
The subject invention further provides a polymer composition consisting essentially of an interpolymer of ethylene and at least one comonomer selected from the group consisting of C3-C20 xcex1-olefins, dienes, and cycloalkenes, wherein the interpolymer is characterized as having:
a. a density of from 0.910 to 0.930 g/cm3,
b. a melt index (I2) of from 0.2 to 10 g/10 minutes,
c. an I10/I2 of from 9 to 20,
d. a molecular weight distribution, Mw/Mn of from 2.1 to 5,
e. a molecular weight distribution, Mw/Mn, as determined by gel permeation chromatography and defined by the equation:
(Mw/Mn) less than (I10/I2)xe2x88x924.63,
xe2x80x83and
f. a gas extrusion rheology such that the critical shear rate at onset of surface melt fracture for the interpolymer is at least 50 percent greater than the critical shear rate at the onset of surface melt fracture for a linear ethylene polymer, wherein the interpolymer and the linear ethylene polymer comprise the same comonomer or comonomers, wherein the linear ethylene polymer has an I2, Mw/Mn and density within ten percent of the interpolymer, and wherein the respective critical shear rates of the interpolymer and the linear ethylene polymer are measured at the same melt temperature using a gas extrusion rheometer.
In an especially preferred embodiment, the subject polymer composition will be characterized as having from one to two crystallization peaks between 45xc2x0 C. and 98xc2x0 C., each having a CTBI of less than 18xc2x0 C., as determined by TREF.